All kinds of granulated grains usually need to be polished at varying degrees. The polishing is to remove the bran powder, tiny moth eggs and peaks (microscopically irregular curves looking like mountain peaks) on the surface of the granulated grains with the use of external force, so that the surface of the granulated grains is clean or is ground (or milled) to be a smooth surface (mirror surface). The grade of products largely depends on the degree of polishing. For example, rice is classified into highly polished rice (such as special grade rice and clean polished rice), lowly polished rice (such as first-grade rice and second-grade rice), unpolished rice and brown rice, the latter three of which usually need moderate polishing. Take the polishing of brown rice as an example, only the bran powder and tiny moth eggs on the surface need to be polished away, with the original color and luster of the rice remained; it is the same for coarse cereals such as wheat, corn and soybean (excluding potatoes). Therefore, the degree of polishing on granulated grains varies with the popularization of nutrition knowledge. As far as rice is concerned, the highly polished rice previously sought after by consumers has become more and more unpopular, while brown rice, unpolished rice and lowly polished rice have become a favorite that consumers rush to buy, as if they buy a local dish. The three kinds of rice have respective shortcomings. With the bran husk and germ totally preserved, brown rice has full nutrients and bright colors (for example, black, red and purple), and is welcomed by special people; besides, it has high yield and high processing benefit. However, as the bran husk and germ are rich in crude fiber, bran wax and furfural, brown rice does not taste good, thus having its sales volume limited. Yet, brown rice can avoid mildew and insects, thus being conducive to storage and prolonging the guarantee period or shelf life of the sugar rice. Unpolished rice is unpolished sugar rice. As sugar powder and tiny moth eggs remain on the surface of unpolished rice, unpolished rice is very likely to have moths and mildew. Therefore, it is not popular despite its low price. Compared with brown rice, lowly polished rice has its remaining of bran reduced by 99% on the surface and by ⅔ increases, thus having better taste. Compared with highly polished rice, lowly polished rice contains more bran husks and fewer germs, thus being more nutritional. Besides, due to its lower processing degree, lowly polished rice has higher yield and better processing benefit than highly polished rice.
To meet the moderate polishing requirements of these granulated grains, the design of polishing equipment should fully consider the characteristics of the granulated grains like rice, such as low anti-shear force, certain anti-extrusion force and relatively high anti-friction force; during the polishing process, the granulated grains should completely avoid the shear force, the extrusion force on them should be minimized, and the friction force on it should be increased, so as to cut down the occurrence of broken rice and improve the yield.
In the prior art, the polishing equipment is an axially propelling roll structure. For example, a spirally propelling iron roll polisher which is frequently used, its working principle lies in that a spiral propeller rolls and drives bunchy granulated grains into a flexible polishing chamber composed of a polishing roller and a rice sieve for polishing. During the process when the granulated grains are propelled and polished, the spiral propeller, the iron roll, the rice sieve and the grains produce rigid extrusion, shear and friction that strongly smoothen the surface peaks of the grains; thus, the surface of the grains can achieve a mirror surface effect (like clean polished rice and bright rice). It can be seen that the roll polisher in the prior art can meet the requirement of highly polished rice with high polishing degree. However, when the machine is used to produce brown rice and lowly polished rice, as the grains can not completely be protected from shear force, nor can its extrusion force be reduced and friction force be increased during the polishing process, the skin of the brown rice will be seriously impaired (commonly known as “skin injury”); about 50 of brown rice will become “impaired rice”, and the original special color of brown rice will not be maintained. To ensure the surface quality of the brown rice, some manufacturers who are restricted by the polishing equipment cannot but use a spirally propelling roll polisher to forcibly polish the brown rice and get rid of the “impaired rice” massively produced during the polishing process by a color sorter. Such method largely reduces the yield and largely increases the processing cost of brown rice. A strange thing thus happens that it is more costly (over double times costly) to polish brown rice than lowly polished rice. As the lowly polished rice contains a great amount of bran powder on the surface, it is very likely to roughen and mildew; therefore, it is not conducive to storage, its guarantee period or shelf life is reduced, and certain polishing is needed. As previously mentioned, it can only be processed by a spirally propelling roll polisher. Thus, the polishing process not only removes the surface bran powder, but also inevitably polishes away a part of endosperms, thus greatly lowering the yield and nutritive value. Under the combined action of extrusion force, shear force and friction force, the polishing process will produce about 0.5%-1% of bran powder (to be blown away by strong air) and 1%-2% of broken rice.
On the other hand, to produce great extrusion force, shear force and friction force during the working process, the spirally propelling roll polisher in the prior art needs high power configuration and will consume too much energy. For example, a 6.5 TPH spirally propelling roll polisher usually needs to have a 75 KW motor, which will consume 11 Kwh/t of power for polishing.
Persons skilled in this art have made efforts to solve the problem of high broken rice rate. For example, a Chinese patent application, with the application number of 932378048, discloses a rice polisher, which is provided with a rice brushing chamber and a rice polishing chamber that are axially connected and are respectively provided with a spiral propelling head. To reduce the extrusion force on the rice grains, the rice grains are conveyed with the help of an oblique bar on a rice brushing roll and a polishing belt on a polishing roll. It says the broken rice rate is less than 1%. Yet, as the work of the spiral propelling mode on the rice grains is still rigid extrusion by the polishing roll, the problems of high broken rice rate and high energy consumption still exist. Therefore, under the new market conditions, it is a subject for the person skilled in the art to find a solution to the three technical problems of “skin injury”, low yield and high energy consumption existing in the polishing equipment of the prior art.